Miniature cameras are becoming increasingly common in mobile electronic devices such as smartphones. There is a constant drive to improve performance of such cameras, while still maintaining the same envelope. Demands on improvements to performance of such miniature cameras are constant, as are demands for continued miniaturization, given the added features and devices added to such mobile electronic devices. In particular, high image quality requires the lens motion along the optical axis to be accompanied by minimal parasitic motion in the other degrees of freedom, particularly tilt about axes orthogonal to the optical axis. This requires the suspension mechanism to be stiff to such parasitic motions. However, given the need to control the lens position with a resolution of 1 micron, such suspension mechanisms must account for friction. Further to this, there is a strong desire, for a given size of camera, to fit bigger lenses and image sensors to improve image quality, and hence there is a desire to reduce the size of components such as actuators.
One feature augmentation that is now standard in such miniature cameras is autofocus (AF) whereby the object focal distance is adjusted to allow objects at different distances to be in sharp focus at the image plane and captured by the digital image sensor. There have been many ways proposed for achieving such adjustment of focal position, however most common is to move the whole optical lens as a single rigid body along the optical axis. Positions of the lens closer to the image sensor correspond to object focal distances further from the camera.
The incumbent actuator technology for such cameras is the voice coil motor (VCM). The VCM technology, as compared to other proposed technologies, has the key advantage of being simple, and therefore being straightforward to design. For such actuators, a current carrying conductor in a magnetic field experiences a force proportional to the cross product of the current in the conductor and the magnetic field, this is known as the Lorentz force. The Lorentz force is greatest if the direction of the magnetic field is orthogonal to the direction of the current flow, and the resulting force on the conductor is orthogonal to both. The Lorentz force is proportional to the magnetic field density and the current through the conductor. Coils of the conductor are used to amplify the force. For actuator operation, either the magnet (or more typically magnets) or the coil is mounted on a fixed support structure, while the other of the magnet (or magnets) or coil is mounted on the moving body, whose motion is being controlled by the actuator.
Successful actuators have been designed both ways around (i.e., with the magnets fixed or the coil fixed), however, the more usual configuration is where the magnets are fixed, and the coil is moving. Representatively, the coil is mounted around a lens carrier or, in some cases, the lens itself. This is the most desirable configuration because the relatively heavy magnets are stationary, and hence their inertia can be avoided. The moving lens carrier is attached to the fixed support structure by an attachment mechanism that allows the lens carrier to move substantially along the optical axis, without parasitic motions, while resisting the Lorentz force of the actuator. In this way the Lorentz ‘force’ is translated into a lens carrier ‘displacement’ by the attachment mechanism.
Another feature augmentation that is desirable in miniature cameras is optical image stabilization (OIS). OIS is a mechanism that stabilizes an image, which may be unstable due to user handshake, by varying the optical path to the sensor. The incorporation of OIS into current miniature camera VCM actuator architecture, however, has been impractical due to compromises between size, power and performance.